Here’s an idea for adding some variation in M+ (or maybe an affix):
Every zone now has a bunch of rare monsters. Put one random somewhat stronger version, same special abilities, of those zone rares in an M+ dungeon of the same zone. Put it somewhere that can be obviously skipped. If the group chooses to do that rare boss the group gets some time taken off their clock and the items on their loot table gets added to the dungeon’s loot table. This could increase the interest in rerunning dungeons.